Directional audio systems work by spatially filtering received (or transmitted) audio so that sounds received (transmitted) along the steering direction are amplified and sounds received (transmitted) along other directions are reduced. The reception or transmission of sound along a particular spatial direction is a classic but difficult audio engineering problem. One means of accomplishing this is by use of a directional array of transducers. It is well known by those skilled in the art that a collection of transducers can be treated together as an array to be combined in engineered ways to spatially filter (either when transmitting or receiving) directional sounds at the particular location of the array over time. The classic means of spatial filtering consists simply of manipulating the constructive and destructive interference pattern of the various sounds that pass through the array using some engineered combination of transducer types, array geometry, time delays, phase delays, frequency filtering, amplitude filtering, and temporal filtering to create a directional interference (a.k.a. directivity) pattern.
Similarly, it is known that waveguides can be used to amplify or otherwise shape sounds traveling through them, as is accomplished in musical instruments for example. Limited scenarios for directional reception and transmission of sound have been addressed by prior devices, such as parabolic dishes, shotgun microphones, microphone arrays, and loudspeaker arrays. A variety of problems remain for prior devices.
In particular, prior array devices can be expensive to manufacture and power particularly if the steering angle of their reception (or transmission) pattern deviates from perpendicular to the plane of the array of transducers (a.k.a. broadside). This is because all audio channels are typically captured (or rendered) independently and simultaneously and steering of the array directivity pattern is accomplished by adjusting digital delays in each audio channel so that the directivity pattern of the array “points” in the correct spatial direction. As a result, signals along a preferred spatial direction are then reinforced and signals along other directions are reduced. This ability to steer the reception (or transmission) directivity pattern from the perpendicular by inserting digital delays into the audio channels is extremely useful, but involves significant levels of additional complexity, manufacturing cost, noise susceptibility, size, weight, and power. These difficulties also scale with the number of channels, quickly making the construction and operation of large steerable, array devices impractical. Many attempts have been made over the years to devise lower cost and robust ways to insert synchronized time delays into multiple audio channels but these attempts have been met with limited success.
A simpler alternative to employing array techniques to construct directional audio systems involves the use of parabolic dishes that reflect audio to or from a single transducer, as appropriate. Parabolic dish techniques are inherently very power efficient and, if of sufficient size relative to the lowest frequency of acoustic signal that the system is designed to handle, highly directional. However, parabolic dishes cannot be steered off the broadside spatial direction except by physically re-orienting the dish. The parabolic dish approach also inherently places the transducer at the focal point of the parabola—out in the environment where the transducer is subject to potentially bothersome effects. Parabolic dishes are also constrained in their shape and cannot be readily adapted for different fixtures or hosts.
What is needed, therefore, is a highly directional audio collection or production system that can operate in a wide range of environments and be applied to various fixed, portable, and mobile applications, is physically and electrically robust, power efficient, economical to manufacture and operate, steerable, inherently scalable, light weight, steerable from its broadside, noise and environment immune, and capable of being installed in fixtures which have non-planar surfaces. Previous implementations of analog and digital microphone and loudspeaker arrays have not been able address all of these concerns simultaneously.